Before the holiday became so commercialized, the traditional Christmas season (as opposed to the Christmas shopping season) began on Christmas Eve. We are currently in the season which is traditionally known as Advent.
We put our tree up on Christmas Eve, and we keep it up after Christmas for the traditional Twelve Days of Christmas, ending on January 6, the Feast of the Epiphany (the coming of the Magi).
I don't let department stores tell me when the Christmas season happens. Theirs ends when the traditional one begins.